Mixamo adding a weapon to character

Does anyone know how to add a weapon to an animated mixamo mesh? I added a weapon to my character in Blender and it plays fine but when I export it as a .glb and try to play the animation in Three.js the weapon just stays still and doesn’t update with the hands. I attached the .blend file if anyone wants to take a look.

I’m testing out the animation using the gltf loader https://gltf-viewer.donmccurdy.com/
terroist.blend (3.2 MB)

Does it have to be a weapon? Would a tool, toy or some other object do?

Yes anything will do. Just need to have the object in the hand when animating. Not sure why it work in Blender but not Three.js